Project Fox Driveshaft - Getting Shafted

For assistance in solidly linking together Project Fox's stout Phoenix TH400 trans and built 8.8-inch rearend, we called up Strange Engineering for some advice. After taking a few quick measurements and discussing the needs of our particular application with Strange, they got busy building us a custom 3-inch chrome-moly driveshaft and had it on our doorstep in less than a week. From our discussion with Strange, it became very obvious that a driveshaft is more than a simple piece of round tubing spinning inside a trans tunnel. There's actually quite a bit of science involved when it comes to minimizing vibrations, and maximizing strength and durability. Selecting the right driveshaft for any application requires understating how tubing diameter, material, and wall thickness-in addition to U-joint and yoke design-all impact driveshaft dynamics and strength. Rather than merely stabbing our new Strange driveshaft into Project Fox and marveling in our ability to pull off such a challenging feat, we'll take a crack at explaining the technical aspects of driveshaft design.

While the car was on the lift, we figured it was also a great opportunity to set up the pinion angle. The procedure for dialing in pinion angle differs depending on rear suspension design, but nonetheless, it can be accomplished with a few simple handtools and is critical for keeping rearend wrapup in check under acceleration. As always, thanks to the good folks at Bill Buck Race Cars in Austin, Texas, for helping us out.

Critical Speed
The rpm at which a driveshaft becomes unstable is referred to as its critical speed. This instability causes a driveshaft to bend in the center like a jump rope, and prolonged operation at critical speed will eventually lead to parts failure. The formula for calculating critical speed is extremely complex, but suffice it to say that it's a function of driveshaft diameter, length, wall thickness, and the modulus of elasticity of the material it's made from. Generally, the shorter the length and the larger the diameter of a driveshaft, the higher its critical speed will be. Although there isn't much you can do about the length of driveshaft your application requires, high-performance aftermarket driveshafts are commonly available in 3-. 3.5-, and 4-inch diameters. The bigger the better, but there is a practical limit to how large you can go due to trans tunnel clearance. As far as driveshaft material is concerned, carbon fiber offers the highest critical speed, followed by aluminum, and then steel.

Strength
While critical speed is indicative of potential driveshaft failure due to prolonged high-speed operation, it doesn't necessarily reflect the strength of a driveshaft. The sheer abuse a driveshaft can handle is primarily attributable to the tubing material. The typical mild steel driveshaft used in many production cars can fail at power levels as low as 400 hp. High-performance aluminum driveshafts are extremely popular upgrades for muscle car enthusiasts due to their high strength and low mass, as they can survive loads up to 1,000 hp. The strongest material by far is DOM chrome-moly, which is often the choice of extreme-duty drag cars producing in excess of 2,000 hp. This strength comes with a weight penalty, however, which also increases parasitic driveline loss. Carbon fiber is the wild card of the lot. Some people claim that carbon-fiber shafts can support 800-plus horsepower, while others have reported failure at substantially lower power levels. Furthermore, while carbon fiber weighs next to nothing, it can also cost twice as much as a comparable chrome-moly driveshaft.

Pinion Angle
If your chassis is already hooking up hard out of the hole, chances are that there isn't much to be gained by changing the pinion angle. In essence, dialing in the right amount of pinion angle prevents a loss of traction rather than enhancing traction. As the driveshaft applies torque to the ring gear, it forces the top of the rearend housing to rotate rearward, and the bottom to rotate forward. If viewed from the passenger side of the car, the rearend naturally rotates counterclockwise under acceleration. Excessive rearend wrapup can unload the rear suspension, compromising grip. Pointing the pinion downward in relation to the driveshaft-also known as negative pinion angle-compensates for this effect. Having the right amount of pinion angle can prevent a loss of traction, but excessive amounts won't improve grip, and increases U-joint wear and parasitic driveline loss. "More negative pinion angle doesn't always give you extra bite, and how much angle a car needs depends on the suspension setup. The goal is to have the pinion in line with the driveshaft under acceleration, which requires dialing some negative pinion angle in when the car is in a static state," Bill Buck says. "With a stock suspension that uses rubber bushings, it might need as much as -7 degrees. Leaf-spring cars have more suspension play, so they need more angle than cars with control arm-style suspension. Cars with urethane bushings need about -4 degrees of angle, while cars with Heim joints need -2 to -3 degrees. An extreme example is Mike Murrillo's Outlaw 10.5 Mustang. Everything is so solidly linked in that car that there's hardly any axlewrap, which means it only needs -1 degree of pinion angle."